The present invention relates to a ratio control system for a continuously variable transmission.
There is known a ratio control system for a continuously variable transmission. Such a ratio control system is disclosed in JP 61-105353. Similar ratio control system is disclosed in U.S. Pat. No. 4,735,113 issued to Yamamuro et al.
Such a continuously variable transmission comprises a V-belt type continuously variable transmission mechanism wherein varying hydraulic pressure supplied to a driver pulley cylinder chamber causes a variation in radius of a running diameter where a V-belt contacts with the driver pulley, thus inducing a variation in a transmission ratio. The hydraulic pressure is adjusted by an electro-hydraulic module including an electronic ratio control unit and a hydraulic control system. The ratio control unit is supplied with various kinds of input information and determines the content of intructions to be supplied to an actuator in response to such various kinds of input information. An output signal of the electronic control unit is supplied to the actuator in the form of a stepper motor for controlling a shift control valve. The shift control valve forms a part of the hydraulic pressure control system and regulates fluid supply to and fluid discharge from the driver pulley cylinder chamber to adjust a hydraulic pressure therein to a value corresponding to a position which the stepper motor takes. The shift control valve does not effect any regulation of hydraulic pressure supplied to a follower pulley cylinder from a line pressure regulator valve. Thus, a line pressure generated by the line pressure regulator valve is kept supplied to the follower pulley cylinder. In the manner mentioned above, a transmission ratio is continuously varied to an appropriate value most suitable for vehicle conditions and running conditions.
However, with the known ratio control system, the transmission cannot downshift at a speed high enough during a transition from an engine drive running state to an engine brake running state. This is because according to this ratio control system, the hydraulic pressure in the driver pulley cylinder is adjusted in accordance with the same ratio shift control maneuver in the engine brake running state as in the engine drive running state. The engine brake running state is initiated when a manual lever is placed at a low range or when the engine begins to be driven by the traction wheels after the accelerator pedal is released. Owing to a considerable slow speed at which the ratio changes, there is a considerable delay until the longitudinal deceleration and the engine speed incline take place, degrading engine brake feel.
That is, in the V-belt type continuously variable transmission mechanism, the relationship between a gripping force which the driver pulley grips the V-belt and a gripping force which the follower pulley grips the V-belt with is a function of transmission ratio and input torque. The relationship is such that the driver pulley needs a gripping force which becomes high as the input torque (in the engine revolution direction) increases and the transmission ratio becomes high.
On the contrary, during engine brake running, the pulley revoltion speed increases at the same time when the input torque increases in the negative direction. Even though the driver pulley requires a less gripping force during engine brake running than during engine drive running, the gripping force which the driver pulley grips the V-belt with becomes stronger than necessary. This is because hydraulic pressure in the driver pulley cylinder is increased due to centrifugal force which the driver pulley is subject to and thus becomes higher than necessary. Therefore, if the hydraulic pressure in the driver pulley is controlled in accordance with the same transient control maneuver during the transient period into the engine drive running, a speed at which the transmission ratio changes in such a direction as to increase the reduction ratio becomes very slow.
An object of the present invention is to provide a continuously variable transmission wherein a good engine brake feel is provided during transient period from engine drive running state to engine brake running state.